Control of an optical fiber scanner

Abstract

Controls for an optical scanner, such as a single fiber scanning endoscope (SFSE) that includes a resonating optical fiber and a single photodetector to produce large field of view, high-resolution images. A nonlinear control scheme with feedback linearization is employed in one type of control to accurately produce a desired scan. Open loop and closed loops controllers are applied to the nonlinear optical scanner of the SFSE. A closed loop control (no model) uses either phase locked loop and PID controllers, or a dual-phase lock-in amplifier and two PIDs for each axis controlled. Other forms of the control that employ a model use a frequency space tracking control, an error space tracking control, feedback linearizing controls, an adaptive control, and a sliding mode control.

Description

FIELD OF THE INVENTIONThe present invention generally relates to controls for an optical scanner, and more specifically, to controls for a resonant optical scanner that is used either for image acquisition or display of an image, wherein the controls determine the movement of a cantilevered distal tip of the optical fiber relative to an adjacent surface.BACKGROUND OF THE INVENTIONThe combination of an optical fiber's low mass, low moment of inertia, and light damping results in large amplitudes or large angular deflections at its tip when excited into resonance. Small movements of the actuator at the base of an optical fiber (base excitation) or weak forces produced by the actuator either along the length of the fiber or at its tip, efficiently result in large amplitudes or large angular deflections at the fiber's tip. When driven to move at resonance or near resonance, an optical fiber scanner can be used for image display and acquisition, as well as the basis of several fiber opti...

AI Technical Summary

Benefits of technology

In one preferred embodiment, the phase control comprises a phase locked loop that varies the phase signal output so as to achieve a predefined relationship between the reference phase signal and the phase of the optical scanner. In this embodiment, the amplitude control comprises an amplitude demodulator that receives the position signal and determines an amplitude of the optical scanner's vibration. A proportional-integral-derivative feedback controller is coupled to the amplitude demodulator and produces the amplitude signal output so as to minimize an error in the amplitude of the optical scanner relative to the amplitude reference signal.

Still another aspect of the present invention is directed to a method for controlling an optical scanner. The method includes steps generally consistent with the functions of the elements comprising the controls discussed above. One preferred form of the method includes the step of producing the drive signal for driving the optical scanner in regard to a first axis and a second axis, where the second axis is generally orthogonal to the first axis. The drive signal produces a desired movement of the optical scanner about only one of the first and the second axes, but includes a component acting on the other of the first and the second axes so as to cancel out an unforced, undesired movement of the optical scanner caused by nonlinear coupling of the axes called whirl.

Where the method uses a model, it further comprises the steps of approximating a continuous control input to the model so as to drive a tracking error in the motion of the optical scanner toward a zero value, and providing a discontinuous control input that is determined as a function of an upper bound on an uncertainty in the model, so that the optical scanner is controlled even though the model is incomplete.

Problems solved by technology

The combination of an optical fiber's low mass, low moment of inertia, and light damping results in large amplitudes or large angular deflections at its tip when excited into resonance.

Small movements of the actuator at the base of an optical fiber (base excitation) or weak forces produced by the actuator either along the length of the fiber or at its tip, efficiently result in large amplitudes or large angular deflections at the fiber's tip.

Combining both high resolution (>100,000 pixels) and wide FOV (>30°) in a single display is a difficult technical challenge, limiting the application of optical scanning for small size, low cost optical scanners that have both high resolution and wide FOV.

To date, a mirror-based resonant scanner fabricated as a micro-electromechanical systems (MEMS) device has yet to be demonstrated as a viable method for manufacturing low cost optical scanners for visual displays of wide FOV and at video scan rates.

Moreover, there is a commercial need for low cost, large-scale (panoramic) optical displays, because larger CRT displays are uneconomical in energy and space.

Finally, the lack of low cost micro-optical scanners with a wide FOV has been the most significant barrier for reducing the size of scanning image acquisition systems for use in surveillance, industrial inspection and repair, machine and robotic vision systems, micro-barcode scanners, and minimally-invasive medical imaging, e.g., a flexible single fiber scanning endoscope (SFSE).

For example, although spiral pattern scans can be implemented efficiently with a very compact scanning optical fiber, it has been found that a spiral pattern of light emitted by a scanning optical fiber is subject to distortion that adversely affects the spiral pattern.

Other that at the fundamental frequency, many of the frequency components of complex scan patterns (e.g., square or triangle waves) are not sufficiently amplified to provide the corresponding complex motion.

A single scanner's resonant properties may change due to environmental effects (temperature changes) or aging (fiber cracking, actuator / fiber coupling deterioration).

Also, optical fibers undergoing large deflections exhibit nonlinear behavior.

This nonlinear behavior produces undesirable scan distortion or inconsistencies.

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